This invention relates to processing shale oil, and in particular to processing shale oil to reduce the arsenic content. Specifically, the invention relates to treating shale oil by a combination of processes-- thermal dearsenation, coking and catalytic hydroprocessing.
Most shale oils produced by conventional retorting processes contain levels of contaminants, such as arsenic, which tend to interfere with subsequent refining or catalytic hydroprocessing operations, such as hydrogenation, denitrogenation and desulfurization. Even if the shale oil is employed directly as a fuel, removal of such contaminants may be desirable from an environmental standpoint. Thus, it is desirable that contaminants such as arsenic be removed, or reduced to low levels, before the shale oil is further processed or used as fuel.
Previous methods for removing arsenic from hydrocarbonaceous oils include contacting raw shale oil with a dearsenation catalyst, such as oxides or sulfides of nickel, cobalt or iron, at an elevated temperature and usually under hydrogen pressure; see, for instance, U.S. Pat. Nos. 3,876,533; 3,933,624; 3,954,603; 4,003,829; 4,046,674; and 4,051,022.
It has also been recognized in U.S. Pat. No. 4,029,571 that arsenic can be removed from shale oil by heat soaking or visbreaking the oil long enough to form a suspended precipitate which must be subsequently mechanically separated from the oil. The oil may also be visbroken and a portion of the visbroken oil catalytically hydrogenated.
In other uses, a thermal treating step has been employed to remove various metallic contaminants from petroleum hydrocarbons, as has been described in U.S. Pat. No. 2,910,343. This reference discloses removal of up to 26 various trace metals, but not arsenic, from a petroleum crude oil feed by non-catalytically reacting the feed with hydrogen in the presence of an inert packing material to form a treated oil of reduced metal content and a solid metal-containing residue. Although the packing may retain a portion of the residue, this reference requires that the treated oil and the remaining residue must be separated by means such as filtration and settling, which are time-consuming and prone to equipment failures. U.S. Pat. No. 3,947,374 discloses removal of the same metals from a hydrocarbon feed by contacting the feed with hydrogen and an inert packing material having a specified pore diameter range to deposit the contaminants on the inert material. U.S. Pat. No. B438,916 discloses demetallation (nickel, vanadium, iron, copper, zinc or sodium but not arsenic) of a residual petroleum fraction by contacting the oil with a refractory oxide in the absence of added hydrogen. These references do not concern arsenic removal or pour point reduction, nor do they recognize that the thermally treated oil is relatively difficult to hydroprocess when compared with the untreated shale oil. Further, they do not suggest a way to improve the hydrogen processability of the oil once it has been thermally treated.
Other methods for treating hydrocarbonaceous oil include visbreaking oil, solvent deasphalting the visbroken oil, and contacting the resulting visbroken deasphalted oil with hydrogen in the presence of a catalyst, as is described in U.S. Pat. No. 3,132,088. In U.S. Pat. No. 2,975,121 there is described a process for removing metals from an asphaltic oil in which the oil is heat-soaked with hydrogen to form a metals-containing precipitate in the oil, and metals-containing oil is solvent deasphalted to remove the metals in the asphaltene fraction.
A combination of visbreaking shale oil followed by hydrogenation of the vapor or lower-boiling portion of the oil and combining the hydrogenated portion with the liquid or higher-boiling portion is taught in U.S. Pat. Nos. 3,523,071 and 3,738,931 as a method of reducing the pour point.
It has also been suggested by Curtin et al in Arsenic and Nitrogen Removal During Shale Oil Upgrading, ACS Div., Fuel Chem. No. 23 (4), 9/10-15/78., to treat raw shale oil by coking it, followed by catalytic hydrodenitrogenation of the coker distillate or a blend of raw shale oil and coker distillate. In this process, however, higharsenic coke distillate was found to have a lower denitrogenation rate than a high-arsenic blend of raw shale oil with coker distillate. This indicates that the coker distillate is more difficult to hydroprocess than raw shale oil.